This invention relates to a circuit arrangement for use with at least one electric accessory load of a motor vehicle.
In modern motor vehicles; the number of electrically operated accessories defining electrical loads and the number of modes of operation of the various loads in a motor vehicle have been considerably increased. For example, typically four modes of operation are provided for the wiper motor of a motor vehicle windshield cleaning system. Namely one mode of operation is for the parking position run of the wiper motor, one mode of operation for slow wiper speed, one for a fast wiper speed and a so-called intermittent mode of operation in which the wiper motor is stopped for a defined period of time after each winding cycle. To some windshield cleaning systems this delay or interval time is firmly and invariably predetermined. In other windshield cleaning systems the interval time can be continuously adjusted through a potentiometer.
This interval time can also be made variable in several steps, wherein, instead of the potentiometer, different fixed resistors are connected with a timing element. In a windshield cleaning system of this kind several different modes of intermittent operation may be achieved through an operating switch. Thus, in a system of this kind the operating switch has several operating positions for the various modes of operation, wherein the different switching signals of the operating switch are conducted to a programming unit which determines the mode of operation determined by the operating switch. In today's windshield cleaning systems an expensive cable with a number of switching signal transmission paths or leads corresponding to the number of different modes of operation is necessary to transmit the switching signal of the operating switch to the programming unit. It can easily be seen that the costs for this lead will increase with the number of different modes of operation. In order to reduce costs circuit arrangements have been devised in which different voltage levers corresponding to the various operating positions are switched onto a switching signal transmission path or lead by the operating switch, and the programming unit has a detector with ranges of detection associated with the different voltage levels and accordingly determines the mode of operation. A circuit arrangement of this kind which includes only one switching signal transmission path between the operating switch and the programming unit is, for example, disclosed the German specification No. DE OS 2,944,224. The amount of circuitry between the operating switch and the programming unit can be reduced considerably by a construction of this kind. It is true, however, that a circuit arrangement of this kind does not always operate reliably. For example, if a short-circuit of the switching signal transmission path or of the input of the programming unit with one or the other poles of the voltage source occurs a voltage level is provided at the input of the programming unit which corresponds with a voltage level of one of the modes of operation. Consequently, the windshield cleaning system can be inadvertently put into operation or a particular unwanted mode of operation can be switched on. A similar fault can also occur during a changeover of the operating switch from one mode of operation to another, because during this period of time no defined potential is switched onto the lead to the programming unit.